Component coating

ABSTRACT

In order to achieve micro alloying of protective coatings for components such as turbine blades in a turbine engine, a base metal is splutter coated by deposition of a trace element to a desired proportion. A protective layer of the base metal is then applied over the trace element to prevent further reaction or oxidation of the trace elements. A coating consumable is therefore formed from the base metal and the trace element. The consumable may be produced immediately prior to coating of the component or may be inertly stored for subsequent use. The consumable is applied by laser deposition techniques whereby a coating is formed in the melting process.

The present invention relates to component coating and more particularlyto coating components with alloys having a low trace element contentwith that trace element being relatively reactive or subject tooxidation.

Clearly, components must be formed in order that they provide thenecessary structural or operational performance in the environmentdictated by the machine or structure within which those components areutilised. In gas turbine engines certain components will operate atrelatively high temperatures and sometimes in excess of the meltingpoint of the metal from which those components are formed. This isachieved through appropriate cooling of the component but also byapplying relatively high performance coatings to those components inorder to achieve resistance to surface pitting at the elevatedtemperatures.

Turbine blades must operate at temperatures in excess of the meltingpoint of the material from which the blade is formed. Typically, aprotective coating is provided particularly at the tips of those bladeswhere the blade is thin and subject to aerodynamic friction. One coatingthat is applied comprises a nickel alloy in which one of the traceelements is hafnium. Previously, this alloy was coated upon the turbineblade component by initially creating a master alloy block in acrucible. This leads to inconsistent and unpredictable losses of traceelements due to reaction with crucible walls. The nickel is heated inthe crucible until it is molten and then the hafnium is added withappropriate stirring, etc in order to achieve or approach the desireddistribution for the hafnium within the master alloy block. It will beappreciated that once melted and the hafnium mixed into the nickel theblock is allowed to cool under controlled conditions, but despite bestefforts it is not possible to obtain a good distribution of hafniumthroughout the master alloy and there is generally a directional aspectto the cooling with cooling being more quickly achieved towards thecrucible walls in comparison with the centre of the molten mass in thecrucible. These problems are a result of trace element concentrationloss of due to reaction crucible walls. These reactions are not usuallypredictable. In any event, hafnium reacts with the ceramic walls of thecrucible, which creates problems including slag within the alloy block.Hafnium also scavenges oxygen from the chamber walls and oxidises andthis again can create occlusions and slag within the master alloy block.As indicated, this approach is not ideal in that the hafnium is poorlydistributed through the master alloy and that alloy may includeundesirable elements. Additionally, formation of the master alloy blockis a relatively massive procedure whereby a large block is formed fromwhich only a small amount may be required at any particular time.

The increased desire to achieve higher engine efficiencies leads toturbine engines operating at higher temperatures and so a greaternecessity to provide reliable and more convenient application ofprotective coatings such as those described above with respect toNickel-Hafnium alloys upon turbine blade components.

In accordance with the present invention there is provided a method ofcoating a component comprising

-   -   a) presenting a base metal to means for ion splutter dispersal        of a trace element;    -   b) spluttering a desired proportion of the trace element upon        the base metal and    -   c) applying a protective layer over the trace element to form a        coated consumable,    -   d) presenting the coated consumable to a component whereby        application of a laser beam causes the coated consumable to melt        in order to apply a coating to the component.

The protective layer is applied to prevent reaction or oxidationdegradation of the trace element. The protective layer may be the sameas the base metal. Generally, the base metal is in particulate form.

Typically, the base metal is the same as the major constituent of thecomponent. Alternatively the base metal has the same composition as thecomponent.

Generally, the coating consumable is presented to the component througha sputter deposition.

Possibly, the base metal is nickel. Possibly, the trace element ishafnium.

Typically, presentation of the base metal in particulate form is in theform of a fluid bed for even distribution of the trace element on theparticulate base metal.

Generally, the component is a blade for a turbine engine.

Additionally, in accordance with the present invention there is provideda consumable for coating or forming a component comprising a base metalupon which a trace element is sputtered and a protective layer isapplied over the trace element. The trace element is susceptible tooxidation when molten. The application of a protective layer over thetrace elements gives a coating consumable which is relatively stableupon the base metal whilst allowing alloying of that trace element withthe base metal and/or with a substrate metal of a component.

Possibly, the consumable is a particulate form, advantageously with amajor dimension in the order of less than 2 microns. Alternatively, thebase metal is in a continuous solid form, such as wire or rod, to allowuse of a deposition technique to form a component.

Additionally, in accordance with the present invention there is provideda component formed from a consumable as defined above.

Further in accordance with the present invention there is provided amethod of forming a consumable for coating or form a component, themethod comprising

-   -   a) presenting a base metal to means for sputter application of a        trace element; and    -   b) sputtering a desired proportion of a trace element upon the        base metal,    -   c) applying a protective layer over the trace element to form        the consumable.

The application of the protective layer prevents reaction or oxidationof the trace element.

Possibly, there is fluidisation of the base metal particulate in apowder bed by mechanical vibration to facilitate application of traceelement evenly throughout the particulate.

Possibly, the base metal is in the form of a wire or rod to allow use ofa deposition technique to form a component.

Possibly, the particulate base metal is electrically charged in order toattract the trace metal.

Embodiments of the present invention will now be described by way ofexample and with reference to the accompanying drawings in which;

FIG. 1 is a schematic illustration illustrating one approach to creatinga coating consumable in accordance with one embodiment of the presentinvention; and

FIG. 2 is a schematic illustration depicting formation of a coatingconsumable in accordance with the present invention as well as means forcoating deposition in accordance with a second embodiment of the presentinvention.

Techniques for weld deposition and additive manufacturing by use ofdirect laser deposition are relatively well known. Essentially a donorpowder or wire is presented to a surface in order that through a laserbeam directed towards the powder there is molten deposition and build upof a surface. Nevertheless, traditionally micro alloying for aerospacealloys in order to provide coating protection is generally performedduring a casting stage whereby there is addition of a master alloycomposition.

Unfortunately trace elements, such as hafnium, can be problematic interms of uncertain yield as these elements tend to be included as meretrace additions which can be lost within the melt either by reaction,substitution with crucible walls or by absorption/adsorption uponfiltration surfaces. The mechanisms thought to apply are based uponoxidation reactions with metal in the molten state in prolonged contactwith ceramic surfaces where the metallic ion component of the ceramic isthermo-dynamically less stable in the ceramic state than the micro alloyadditional trace additive element. This mechanism can also apply wherethe alloying additional trace element forms a ceramic particle but theseparticulates are filtered out by high surface area, ceramic (surfacetension based filters). Finally, when added to a casting chemistry priorto atomisation, there is a limitation of the homogeniality that can beachieved by electromagnetic mixing. Thus, distribution of the tracemetal within the master alloy will be compromised.

As indicated above, particular problems are experienced with respect toapplying coatings for blade tips used in turbine engines. Blade tippingis used to give wear and oxidation resistance coatings to the tips ofturbine blades. The coating is of a different material at the tip inorder to render blade tips as having resistance to higher temperaturesand tip abrasion. The more consistent and homogenous the coatingprovided upon the blade tip generally the more effective protectionprovided. Turbine blades are exposed to extremely high operatingtemperatures. These high operating temperatures are mitigated by coolingair, but the amount of cooling air used in this way is limited andaffects engine efficiency. Greater engine efficiency is gained byachieving high material operating temperatures. The practical limit toblade operating temperatures is the oxidation resistance of the materialfrom which the blades are formed. As blades are oxidised at their tips,material is lost locally causing the geometry of the tip to change. Anychange in blade geometry has two problems. Firstly there is a pressureloss which impairs engine performance and secondly the changed airflowtends to accelerate the rate of chemical erosion of the blades.

It is known to counter the oxidation breakdown by applying a coating toprovide protection at higher temperatures. This coating generallycomprises provision of additive quantities of alloys of similarcomposition to the blade but containing trace elements such as hafnium,yttrium and lanthanum. These elements tend to stabilise the highertemperature protective oxide layer as a passive barrier to oxidation ofthe underlying base metal alloy of the blade.

The quantity of these trace elements required is extremely small. Thetrace elements referred to in the present invention may be any of theelements in Group III or Group IV of the Periodic Table or the group ofelements known as Lanthanides. Generally, the trace element will bepresent in the range of 30 to 80 parts per million to be effective.Below 30 parts per million trace elements there is insufficient quantityof alloy addition to be effective in terms of distribution within themelt. However, above 80 parts a million the parent base metal alloybecomes prone to localised melting where a lower melting point alloy canform within the blade's tip region. Localised melting acts as a limit onthe temperature capability of the blades. Limitations with respect tocurrent alloying techniques creates uncertain yields as a result ofprocess variability when making an addition to the block melt for bladetipping alloys based upon nickel or cobalt alloyed with chromium,aluminium and a trace element such as hafnium, yttrium and lanthanum.There tends to be unpredictability in yield due to the effect of thesetrace elements scavenging oxygen ions from the crucible walls andotherwise interacting with the ceramic liner of crucibles. Oxides formedin this way can be detrimental to downstream processing. Alloying yieldvariability is related to the variation in residence time and cruciblewall variations. In such circumstances, people forming these alloys tendto over alloy, that is to say incorporate relative excessive proportionsof trace elements. However, this process is unpredictable and causeswide variation in the concentration of the trace element in the alloy.Once the alloys have been formed it is generally converted toparticulate form and then utilised in a direct laser deposition processin order to apply the coating to the component.

The present invention retains use of direct laser deposition or asimilar technique in order to apply the coating to the component, butthe powder is formed by a distinct process. Essentially, the traceelement is not added to the base metal alloy during a melt phase withina crucible, but rather that base metal alloy is initially formed as aparticulate powder and the trace elements ion sputtered upon that basemetal alloy in an appropriate environment. That environment generallycomprises a vacuum or inert gas with the sputter electrode for the traceelement appropriately cool in order to inhibit oxidation of the traceelement thereat. In such circumstances, the particulate base metal alloyacts as a carrier for the trace element such that the uncertainties ofalloy melting and casting are avoided. As the proportion of traceelement required is so low, ion sputter deposition upon alloyparticulates allows close control of the proportion of trace elementspluttered upon the base alloy particulate.

Furthermore a protective layer or coating is provided over the traceelement that has been sputtered upon the base alloy. The protectivecoating prevents reaction or oxidation of the trace element andincreases the effective life of the particulate.

The protective coating applied will generally be the same as the basealloy of the underlying particulate. Thus upon deposition on thecomponent the protective layer will become mixed with the baseparticulate forming the micro alloying necessary to create theprotective coating to the component.

It will be appreciated that in providing a protective coating of thebase material that the actual base particulate upon which the traceelement is ion splutter deposited may be reduced so that the overallsize of the coating particulate does not change and more importantly thechemical composition of that coating particulate remains as required formicro alloying.

In a particular embodiment of the present invention the coating takesthe form of a partial line of sight coating through ion plating orsputter of the trace element on the base alloy particulate. The basealloy particulate as indicated is a consumable in the deposition processutilised. In such circumstances, normally the particulate is presentedas a powder, although the powder may be compressed into a wire forpresentation to a component such as a blade tip. When a powder is usedthen the powder is channelled in a discrete trough or groove in a discfor presentation at an even rate with an inert carrier gas. The powderstream is divided into the melt pool upon the component such that theapplication of a laser heats the substrate allowing the incident powderto fuse by melting or sintering to the component surface to establishdeposition and application of the coating layer.

Direct laser deposition generally occurs within an inert gas shield orwithin a vacuum, but in either event some form of feed for theconsumable powder or wire is required. In such circumstances a labyrinthor other seal is necessary through which the consumable platingparticulate is presented.

It will be understood that the sputter of the trace element could followthe deposition of a previous base alloy layer. In such circumstances,micro alloying of the trace element will occur through fusing in withsubsequent deposition passes of the deposition process.

The present invention allows for more consistent processing to highertarget concentration accuracy for the micro-alloying constituents, thatis to say the trace elements. In such circumstances, a higher minimumconcentration level, closer to the optimum micro alloy chemistry levelscan be achieved. In such circumstances the coating applied to componentssuch as blades at their tip can be optimised for less oxidation of thatblade tip and therefore longer life.

Referring to FIG. 1, schematically illustrating application of the traceelement to a base alloy particulate mass 1. This mass is formed by avolume of particulate or powder base alloy grains in the order of a fewmicrons in diameter. In order to facilitate ion deposition, a vacuumchamber 2 is formed about a boat or vessel 3 holding the mass of basemetal 1. A vacuum seal 4 is provided across the vessel 3 with an ionsource 5 presented to splutter to an appropriate composition traceelements upon the particulates of the mass 1. The powder bed can befluidised by mechanical vibration to ensure that all the particles arecoated evenly with trace elements or metal. The chamber 2 is evacuatedby extraction in the direction of arrowhead 6 or has an inertenvironment to avoid oxidation of the ion deposition or other reactionsin the transient before deposition. Furthermore, the ion source 5 iscooled to prevent or at least inhibit such reaction or ionisation of thesource 5.

As the mass 1 is typically relatively cool, it's reactivity is reducedand therefore once an appropriate composition proportion has beendeposited from the ion source 5 upon the mass 1, a protective layer ofthe base metal is applied before presentation to the component for laserdeposition in accordance with accepted procedure. In such circumstances,a conveyor 7 can be provided whereby batches of particulates areprocessed ready for subsequent coating upon components.

FIG. 2 illustrates an alternative approach to providing a depositionpowder for micro alloying and coating upon a component. Thus, a hopper21 is utilised to store base alloy powder typically under a dry nitrogenor vacuum atmosphere. Powder falls from the hopper 21 upon a conveyor 22either continuously or in batch increments such that the powder flows inthe direction of arrowhead 23 to become exposed to an ion source 24. Theconveyor 22 acts to corral and channel the powder base alloy such thatit receives sputter deposition of trace element in the direction ofarrowhead 25. Thus, prior to sputter deposition, the powder comprisessimply the base alloy consumable utilisable for direct laser depositionwhilst after splutter deposition of the trace element that powder 28 hasa short life before oxidation reaction of the trace element. Thus, thepowder 28 is presented to the component.

Presentation of the ion-plated powder is typically directed by an inertshield gas stream such that the powder is deposited upon the component.Generally this occurs within an inert gas shield and the flow will befunnelled and mechanically entrained to achieve appropriatepresentation. The laser melts the powder 28 such that there is microalloying to create a coating to the component as required.

It will be appreciated from the above that sputter deposition of thetrace element upon the base alloy particulates avoids problems withrespect to accurate addition of trace elements to an alloy being formedin a crucible. By applying a protective layer over the trace elementsthe process can occur immediately prior to deposition or alternativelyany coated particulate can be produced for stock and stored underacceptable environmental conditions for use when required.

As described above with regard to FIGS. 1 and 2, the base alloyparticulate may be simply passively presented upon a conveyor orotherwise such that the ion sputter deposition upon the base alloyparticulate is then substantially only upon one side. This may have nosignificant detrimental effects, but alternatively the base alloyparticulate may be presented upon a fluidised bed such that the randommotions of the base alloy particulate then leads to trace elementdeposition more evenly over the particulate surface.

In such circumstances, the powder particulate of the base alloy may bestatically presented or randomly presented in a fluid bed agitator.

Where appropriate, it may also be possible to electro statically chargethe base alloy particulate powder to further assist attraction throughion spluttering of the trace element.

Fundamentally, the present invention presents the trace element by asplutter deposition upon the particulate, but that trace element doesnot become alloyed with the base alloy particulate until melted to formthe protective coating upon the component. It will be appreciatedpreviously a relatively large block of master alloy for the protectivecoating alloy is formed requiring a number of melting stages and a veryhigh energy input to achieve as indicated a variable quality product interms of distribution of the trace element.

Although described with regard to particulate powder, it will beappreciated that the present invention with respect to splutterdeposition of the trace element may be performed upon a fine wire suchthat the wire is again presented to a component in order to form thedesired micro alloy composition.

As indicated above typically a particulate form of base metal isutilised upon which the trace elements are deposited. However, where theblade tipping base metal can be formed into a continuous solid form,such as wire or rod, then some technique of sputtering trace metal orelements upon that solid wire etc can be used. With the wire then usedto provide the blade tip coating etc. Furthermore, by use of acontinuous solid form it will be understood that an entire component maybe formed by deposition techniques from that continuous solid wire orrod with the trace element sputtered thereon then distributed throughoutthe structure. Such deposition techniques include Direct Laserdeposition (DLD) and Shaped Metal Deposition (SMD). However, suchstructures will be expensive as the relatively high cost trace elementis distributed throughout the whole structure rather than just upon thesurface but may be justified where enhancement of physical propertiessuch as heat resistance and strength, etc are required.

1. A method of coating a component comprising a) presenting a base metalto means for ion splutter deposition of a trace element; b) splutteringa desired proportion of the trace element upon the base metal; c)applying a protective layer over the trace element to form a coatedconsumable; d) presenting the coating consumable to a component wherebyapplication of a laser beam causes that coated consumable to melt inorder to apply a coating to the component.
 2. A method as claimed inclaim 1 in which the protective layer is the same as the base metal. 3.A method as claimed in claim 1 in which the base metal is first formedin particulate form.
 4. A method as claimed in claim 1 in which the basemetal is in particulate form and is presented in a fluidised bed inorder to achieve splutter dispersal of the trace element more evenlyabout the base metal.
 5. A method as claimed in claim 1 in which thebase metal is the same as the major constituent of the component.
 6. Amethod as claimed in claim 1 in which the base metal has the samecomposition as the component.
 7. A consumable for coating or forming acomponent comprising a base metal upon which a trace element is ionsplutter deposited and having a protective layer above the trace elementin accordance with a method as claimed in claim
 1. 8. A consumable asclaimed in claim 7 wherein the base metal alloy is a nickel or cobaltbased alloy.
 9. A consumable as claimed in claim 7 wherein the traceelement is hafnium, yttrium or lanthanum.
 10. A consumable as claimed inclaim 7 in a particulate form and having a major dimension in the orderof less than 2 microns.
 11. A consumable as claimed in claim 7 in asolid continuous form to allow formation of a component by a depositiontechnique such as DLD or SMD.
 12. A method of forming a consumable forcoating or forming a component, the method comprising presenting a basemetal to means for ion splutter application of a trace element;spluttering a desired proportion of the trace element upon the basemetal and applying a protective layer over the trace element to form thecoating consumable.
 13. A method as claimed in claim 12 wherein the basemetal is presented in a fluidised bed for more even distribution of thetrace element on the base metal.
 14. A method as claimed in claim 12wherein the base metal is a solid form such as wire or rod to allowformation of a component by a deposition technique such as Direct Laserdeposition or Shaped Metal deposition (SMD).